In the conventional railroad air brake system, as developed from the Westinghouse air brake, there is a brake pipe air line which passes from the lead locomotive and from vehicle to vehicle down the length of the train consist and provides two basic functions.
First, air from the brake pipe is used to charge compressed air reservoirs disposed on each of the railroad cars. The air stored in these reservoirs provides the energy needed to apply the brake shoes when a brake application is required. When the train is running normally, and no brake application is needed, a high pressure, typically about 90 psi for freight trains and about 110 psi for passenger trains, exists in the brake air line. The reservoirs on the cars are charged to the same pressure as the air in the brake pipe. Second, when a brake application is required, air is vented from the brake pipe air line through a valve located in the lead locomotive. This causes the pressure in the brake pipe air line to be reduced by a controlled amount. In the individual cars of the train, this reduction of pressure is used as a signal to apply the brakes. In this event, valving in the cars utilizes the compressed air in the reservoirs to supply air to brake cylinders which, in turn, apply a force to the brake shoes so that the brakes are applied.
Although this conventional air brake system was an enormous improvement over the art prior to it, it nevertheless had some features where improvement was possible. For one thing, the time required for a pressure decrement to propagate down the line of cars in a long freight train is relatively long. For example, the time to complete a full service application would be about a minute for a mile-long train. Hence, when a brake application is required, it takes some time before all of the brakes in the train are applied. This is the case for both normal and emergency brake applications, although emergency applications are completed in less than half the time required for service applications.
There are also some operational difficulties due to the fact that the same compressed air line is used both for charging the air reservoirs in the cars and for signalling a brake application. When a brake application is made, some of the air in the air reservoirs in the cars is depleted. Since the pressure in the brake pipe air line has been reduced to signal the brake application, there is not sufficient air pressure in the brake pipe air line to recharge the air in the reservoirs. The air in the reservoirs cannot be recharged to its initial pressure while the brake line air pressure is low for applying brakes. Another restriction of conventional pneumatic brakes designed for use on very long trains is that they must be of the direct release type. This means that, while the brakes may be applied in stages, when a brake release is made the brakes must be released completely.
One necessary practice which stems from this aspect of traditional airbrake systems is the practice of power braking. This is a case in which an engineer, upon starting to descend a grade, makes a brake application which is too heavy for a section of the grade, so that the train does not maintain its preferred speed. In this case, the engineer may apply engine power to maintain speed. Hence, the brakes and locomotive are working against each other. Fuel is expended and brake shoes are worn.
One method of decreasing the time needed for the brake application signal to reach remote portions of the train is to provide a radio link so that when a signal originates in a lead locomotive to apply brakes, a radio signal is transmitted which is received at some distance down the line of cars. Where the signal is received, it causes local venting of the brake line so that brakes are applied more rapidly. The WABCO EPIC a brake system, for example, may be operated with a radio link for this purpose.
The radio approach may have difficulty due to terrain which intervenes between the locomotive and the remote receiver causing the remote unit to fail to dump brake pipe pressure. Hence, some systems use electrical trainlines which are electric cables connected between cars down the length of the train. These electric cables carry signals to electro-pneumatic valves which vent brake pipe air at many points along the train and, hence, cause a relatively rapid and more uniform brake application.
Either of these approaches, the radio link or the trainline link, can improve the response time of the system. However, the operational difficulty of not being able to partially or gradually reduce a brake application which is in force is not solved by shortening the time needed for brake pipe pressure to drop.